1. Field of the Invention
This invention relates to a process for solvent dewaxing waxy hydrocarbon oils. More particularly, this invention relates to an improved process for dilution chilling dewaxing waxy petroleum oil stocks in a staged chilling zone wherein cold dewaxing solvent is injected into said zone at a plurality of stages therealong and wherein the cold dewaxing solvent and the waxy oil are substantially instantaneously mixed in each stage as the waxy oil-solvent mixture passes from stage to stage, the improvement which comprises using a dewaxing solvent selected from the group consisting of a mixture of (a) methylene chloride and (b) acetone, methylethyl ketone, methanol and mixtures thereof, and wherein the methylene chloride comprises at least about 20 LV% of said solvent. This invention is particularly useful for dewaxing lubricating waxy oil stocks.
2. Description of the Prior Art
It is well known that wax-containing petroleum oil stocks can be dewaxed by shock chilling with a cold solvent. It is also known that shock chilling, in itself, results in a low filtration rate of the dewaxed oil from the resultant wax/oil-solvent slurry. Because of this, the conventional method of solvent dewaxing wax-containing petroleum oil stocks has been by cooling in scraped surface heat exchangers using an incremental solvent addition technique. In this technique, the dewaxing solvent is added at several points along the chilling apparatus. The waxy oil is chilled without solvent until some wax crystallization has occurred and the mixture is thickened considerably. The first increment of solvent is introduced at this point and cooling continues. Each incremental portion of solvent is added as necessary to maintain fluidity until the desired filtration temperature is reached at which point the remainder of the solvent required to obtain the proper viscosity of the mixture for filtration is added. In using this technique it is well known that the temperature of the incrementally added solvent should be the same as that of the main stream of oil at the point of addition to avoid the shock chilling effect. This process shall be hereinafter referred to as Conventional Incremental Dilution Dewaxing for the sake of brevity.
It is now well known that the adverse effects of shock chilling can be overcome by introducing the waxy oil into a staged chilling zone and passing the waxy oil from stage to stage of the zone, while at the same time injecting cold dewaxing solvent into a plurality of the stages and wherein a high degree of agitation is maintained in the stages so as to effect substantially instanteous mixing of the waxy oil and solvent. As the waxy oil passes from stage to stage of the cooling zone, it is cooled to a temperature sufficiently low to precipitate wax therefrom without incurring the shock chilling effect. This produces a wax/oil-solvent slurry wherein the wax particles have a unique crystal structure which provides superior filtering characteristics such as high filtration rates of the dewaxed oil from the wax and high dewaxed oil yields. The basic concept of dilution chilling dewaxing referred to in this invention is disclosed in U.S. Pat. No. 3,773,650, the disclosures of which are incorporated herein by reference. This process shall hereinafter be referred to as DILCHILL for the sake of brevity.
A number of improvements and modifications have been made to the basic concept of DILCHILL. U.S. Pat. No. 3,642,609 shows that in a vertically staged cooling tower, the velocity of the solvent at the injection points within each stage should be at least 5-30 times that of the peripheral velocity of the mixer blades. This results in greater filtration rates and higher dewaxed oil yields than could otherwise be obtained without the relatively higher velocity solvent injection. In U.S. Pat. No. 3,775,288 is disclosed a combination of dilution chilling with scraped surface chilling for dewaxing lubricating oils. U.S. Pat. No. 3,681,230 discloses adjusting the dewaxing solvent composition so that the waxy oil and solvent are immiscible near the last stage of the cooling zone. This results in superior dewaxed oil yields and higher filter rates when the waxy oil stock being fed to the tower is relatively high in viscosity and molecular weight. U.S. Pat. No. 3,850,740 discloses partially prediluting the waxy oil when same is a relatively heavy feed, such as a resid, or a brightstock, before the oil is introduced into the chilling zone.
The use of ketones and mixtures thereof as well as mixtures of ketones and aromatic and/or aliphatic hydrocarbons in solvent dewaxing is as well known in the art as is the use of autorefrigerant solvents and halogenated solvents. The Di-Me solvent dewaxing and wax deoiling process is a commercially successful solvent dewaxing process employing a binary solvent mixture of dichloroethane and methylene chloride. Mixtures of ketones such as methylethyl ketone (MEK) and methyl isobutyl ketone (MIBK) as well as mixtures of ketones and aromatic hydrocarbons such as MEK/toluene are also well known and are used commercially all over the world, as are autorefrigerants and mixtures of autorefrigerants and ketones such as propylene and acetone. Although the use of mixtures of methylene chloride and other solvents for solvent dewaxing has been suggested many times in variouus patents and articles, such a combination has never met with any commercial or practical success other than the Di-Me process using mixtures of dichloroethane and methylene chloride.
One of the eariest disclosures on the use of methylene chloride may be found in U.S. Pat. No. 1,978,010 which is directed towards the use of methylene chloride alone or in admixture with other wax solvents such as acetone, butanol, benzol or naphtha for solvent dewaxing waxy oils. Another early disclosure on the possibility of dewaxing lubricating oils with methylene chloride alone or in admixture with a ketone dewaxing solvent may be found in an article by P. J. Carlisle and A. A. Levine entitled "Dewaxing Lubricating Oils with Methylene Chloride" in Vol. XXIV, No. 4 of Industrial and Engineering Chemistry (pages 384-387, 1932) wherein the use of methylene chloride alone and in admixture with acetone is disclosed as a potential dewaxing solvent. However, this did not meet with any commercial success and, on page 417 of their book entitled "Petroleum Refining with Chemicals" (Elsevier, Amsterdam, 1956), Kalichevsky and Kobe recite, in reference to the 1932 article by Carlisle and Levine, that in extracting a Pennsylvania lubricating oil fraction with a mixture containing 40% methylene chloride and 60% acetone, the filter rates were low. Kalichevsky and Kobe also stated that such solvents (mixtures of methylene chloride and acetone), when used for the liquid phase extraction (precipitation) of the wax, are not practical because of very high oil losses. Therefore, the prior art teaches a complete lack of success in solvent dewaxing using a dewaxing solvent comprising methylene chloride alone or in combination with other solvents, the one exception being the dichloroethane-methylene chloride mixture of the Di-Me process. Hence, with the exception of Di-Me, the art teaches away from the use of methylene chloride for solvent dewaxing.